Alkali-activated binders produced from phosphate-derived clay and natural pozzolan: effect of precursor composition on phase evolution, microstructural development and mechanical performance

This study investigates the alkali activation of thermally treated Moroccan yellow clay derived from phosphate mining residues and natural volcanic pozzolan from the Middle Atlas as sustainable aluminosilicate precursors for geopolymer binder production. Following calcination, calcined yellow clay (YCC) and calcined pozzolan (CP) were combined in different proportions, while all activation parameters were kept constant to isolate the influence of precursor composition on phase evolution, microstructure development, and mechanical performance. Mineralogical and microstructural analyses reveal that geopolymerization is strongly governed by the CP/YCC ratio under fixed activation conditions. XRD results indicate the destabilization of reactive aluminosilicate phases and the formation of amorphous binding products associated with C-A-S-H/N-A-S-H-type gels, whereas persistent pyroxene-related crystalline phases in CP exhibit limited reactivity and increasingly act as inert structural components at higher replacement levels. SEM observations confirm a progressive transition from dense and homogeneous matrices to more heterogeneous and porous microstructures with increasing CP incorporation. This hierarchical evolution directly governs compressive strength, with the optimum formulation (90 wt% YCC and 10 wt% CP) achieving 36.2 MPa at 60 days of curing, demonstrating that limited CP incorporation promotes an optimal balance between precursor reactivity, gel formation, and matrix densification under fixed activation conditions. Beyond this threshold, increasing CP content progressively reduces gel continuity and matrix connectivity, resulting in a monotonic decline in mechanical performance. Overall, the results identify the CP/YCC ratio as a key compositional design parameter for engineering low-carbon alkali-activated binders derived from Moroccan geological resources.